Intracellular levels of phosphorylation are regulated by the coordinated action of protein kinases and phosphatases. Somatic mutations in the PTEN (Phosphatase and Tensin homolog deleted on chromosome 10) gene are known to cause tumors in a variety of human tissues. In addition, germline mutations in PTEN are the cause of human diseases (Cowden disease and Bannayan-Zonana syndrome) associated with increased risk of breast and thyroid cancer (Nelen M R et al. (1997) Hum Mol Genet, 8:1383-1387; Liaw D et al. (1997) Nat Genet, 1:64-67; Marsh D J et al. (1998) Hum Mol Genet, 3:507-515). PTEN acts as a tumor suppressor by regulating several signaling pathways through the second messenger phosphatidylinositol 3,4,5 triphosphate (PIP3). PTEN dephosphorylates the D3 position of PIP3 and downregulates signaling events dependent on PIP3 levels (Maehama T and Dixon J E (1998) J Biol Chem, 22, 13375-8). This inhibits downstream targets mainly protein kinase B (PKB/AKT). PTEN sequence is conserved in evolution, and exists in mouse (Hansen G M and Justice M J (1998) Mamm Genome, 9(1):88-90), Drosophila (Goberdhan D C et al (1999) Genes and Dev, 24:3244-58; Huang H et al (1999) Development 23:5365-72), and C. elegans (Ogg S and Ruvkun G, (1998) Mol Cell, (6):887-93). Studies in these model organisms have helped to elucidate the role of PTEN in processes relevant to tumorigenesis. In Drosophila, the PTEN homolog (dPTEN) has been shown to regulate cell size, survival, and proliferation (Huang et al, supra; Goberdhan et al, supra; Gao X et al, 2000, 221:404-418). In mice, loss of PTEN function increases cancer susceptibility (Di Cristofano A et al (1998) Nature Genetics, 19:348-355; Suzuki A et al (1998) Curr. Biol., 8:1169-78).
AKT signaling is frequently hyperactivated by a variety of mechanisms in a wide range of human cancers, including melanoma, breast, lung, prostate, and ovarian tumors (see Vivanco I and Sawyers C L (2002) Nat Rev Cancer. 2(7):489-501; Scheid M P and Woodgett J R (2001) J Mammary Gland Biol Neoplasia. 6(1):83-99). In tumor cells, the AKT protein kinase activity can be elevated by amplification and overexpression of the AKT2 gene, or by increased production of phosphatidylinositol (3, 4, 5) trisphosphate (PIP3), which activates AKT by recruitment to the plasma membrane. In normal phosphoinositide metabolism, phosphatidylinositol (3, 4) bisphosphate (PIP2) is phosphorylated by phosphatidylinositol 3-kinase (PI3K) to generate PIP3, and PIP3 is dephosphorylated back to PIP2 by the lipid phosphatase PTEN. Most commonly, however, PIP3 levels in tumor cells are elevated by mutation or deletion of the PTEN tumor suppressor, at rates as high as 40-50% of prostate cancers.
The PTEN/AKT pathway promotes tumor progression by enhancing cell proliferation, growth, survival, and motility, and by suppressing apoptosis. These effects are mediated by several AKT substrates, including the related transcription factors FKHR and AFX, for which phosphorylation by AKT mediates nuclear export. Signaling through the TOR (mTOR) branch of the PTEN/AKT signaling pathway regulates protein synthesis, which is directly involved in the growth activation and cellular transformation properties of AKT signaling. TOR directly phosphorylates several targets including 4EBP1 and p70S6 kinase. p70S6 kinase directly phosphorylates ribosomal protein S6 (RPS6) (Bader A G et al. (2004) Oncogene 23:3145-3150; Hay N et al. (2004) Genes Dev. 18:1926-1945). Additional direct AKT substrates have been identified which can serve as a readout for PTEN/AKT signaling activity, including the protein PRAS40 (Kovacina K S et al. (2003) JBC 278(12): 10189-10194).
Identification of the involvement of novel genes in particular pathways, such as disease pathways, and their function in such pathways can directly contribute to the understanding of modulation of these pathways. Further, the identified genes may be attractive candidate targets for novel therapeutics.
All references cited herein, including patents, patent applications, publications, and sequence information in referenced Genbank identifier numbers, are incorporated herein in their entireties.